The present invention relates to means to improve the quality of film produced by a blown film extrusion line.
As plastic resin is extruded from a heated extruder having an annular die, the molten plastic resin is pulled away along the die axis in the form of an expanded bubble. After the resin cools to a set diameter as a result of application of cooling air by an air cooling ring, the bubble is collapsed and passes into nip rolls for further manufacturing steps.
As the film is extruded, thickness variations occur about the circumference of the bubble. It is recognized that these variations are caused by the melting, forming and cooling processes which are carried out by the extruder and its screw, the die and the air ring, respectively, as well as by variations in the amount of cooling that the film receives as it expands and is pulled toward the nip rolls.
In general, the thickness variations create problems for subsequent downstream conversion equipment such as printing presses, laminators, or bag machines. In processes where the film is wound onto a roll prior to converting, the thicker areas can build up on top of each other and thereby create hills and valleys on the rolled surface. Uneven rolls can further reduce the effectiveness of the downstream equipment. Any slack or tightness originally associated with gauge variations are magnified.
The tension variation across the film can also create creases in the film which can subsequently affect other processes. For example, in a printing press, ink will not transfer to the film on the inside of a crease and thus the quality of the finished product is degraded. In bag machines, if a crease happens in a location where a seal is placed, the seal will be defective. Further, the seal and perforation quality in the film is highly dependent on film tension and thickness and therefore any unexpected variations can result in poor quality. To avoid these problems, blown film manufacturers have used spreader rolls or other means to eliminate creases. These means complicate and slow down the manufacture of the finished product.
It is generally desired to obtain high quality film during the extrusion process so that the downstream equipment can be run faster and obtain higher quality products. Manufacturing processors have recognized that by eliminating gauge variation (film wall thickness) during the extrusion process, one is able to obtain higher quality film products.
Manufacturing processors primarily rely on equipment suppliers to provide extruders, screw and die design technology to limit gauge variations. This typically yields an average of + or -5 to 10% variations in gauge directly attributable to the processing equipment. These variations take the form of several equally spaced gauge bands or port lines around the circumference of the plastic film tube. The number of bands directly relates to the number of flow channels within the die. The magnitude of variation for each band is related to how well the plastic melt is distributed by its associated flow channel. This varies from channel to channel since the melt viscosity entering the die is typically nonuniform and is a function of the extruder and die design. Another form of die related variation can be caused by non-uniform annular gaps or lips through which the polymer exists. These gaps are usually adjustable and depend on the manufacturing operator's skill and feedback control to minimize associated thickness variations.
The magnitude of port line variation can be modified and reduced by proper application of a cooling system. It is understood that the shape a bubble takes during the cooling process significantly impacts a film property such as gauge variation associated with the melting and forming equipment. There are several cooling systems which serve to shape the bubble to shapes other than that which would occur without any influence.
One such method involves stacking multiple air rings on top of each other which are spaced apart and encompass and shape the bubble. This design has a significant disadvantage in that the stacked air rings seriously narrow the range of bubble diameters and thus film widths that can be produced when compared with a standard single ring process. If sizes outside of the operating range of the stacked air rings are desired, the line must be shut down and the equipment must be changed to accommodate the different size film width. Another disadvantage occurs during start up of the extrusion line when direct access to the die area is required so that the processor can reach the molten film issuing from the lips. Stacked air rings such as discussed above, significantly impair this access.
Another known cooling method involves placing fixed diameter plates or irises a distance above and generally sealed to the air ring. The bubble runs inside a sealed chamber up to an open plate/iris diameter (bubble diameter typically runs approximately two inches smaller). As plate/iris diameter changes are made, the bubble follows it and changes shape. The disadvantage of such plates/irises is that significant turbulence is induced as air flows through the gap between the plate/iris and the bubble which degrades stability especially at diameters that alter the natural shape of the bubble. A further disadvantage of this method is that the plate/iris has limited access to the die lips during line start-up.
Cooling equipment also causes film thickness variations which add to those from the melting and forming equipment. A most significant variation is when non-uniform air is drawn into the cooling air stream from the surrounding atmosphere adjacent the extrusion line. Atmospheric air is non-uniform in many properties including temperature, flow, and humidity, since there are large, high temperature equipment which are used in the process. The heat which discharges from this equipment affects the air surrounding the extruded resin and can cause thickness variations of + or -15%. These variations occur since large volumes of air are aspirated from the atmosphere by high velocity air exiting from the cooling ring adjacent the base of the bubble.
A further cooling problem exists based on the time of day as well as seasonal variations in ambient atmospheric conditions. These significantly impact the operation of a line and especially affect the throughput rate which can change by 10% or more. Presently, manufacturers have had only limited success in controlling ambient air variation. The crudest and most widely practiced attempt at controlling ambient air variation is by the use of fans and barriers placed strategically around the process to compensate for temperature variations. The main problem with this approach is that the ambient conditions are constantly changing requiring barrier and fan repositioning. Additionally, seasonal changes are not compensated at all. The diameter plate/iris chambers also have limited success in controlling atmospheric variation of this type since a portion of the molten film remains outside of the influence of the chamber and ambient air is aspirated into the exiting air stream.
A further method previously employed to control ambient air variation is by physically enclosing the process starting from the top of the air ring and extending upward for several feet. This approach has varying degrees of success since these systems do not seal at the top of the bubble. Ambient air is typically drawn over the top and is aspirated into the cooling air stream. An additional problem is that this approach limits access of personnel to the film during line start-up.
More sophisticated systems actively measure the gauge of the film on-line through closed loop control of localized die lip temperature or the air flow just above the die lips. These systems typically attempt to compensate for film thickness variations. The major drawback of the system is that gauge corrections depend on the accuracy of the on-line gauge sensor, any misreading of the actual thickness will cause an inaccurate correction and hence will result in film of unacceptable quality. These systems are also complex and expensive and require significant training of manufacturing operators and maintenance crews.